Add streaming_diff crate (#23264)

This PR extracts the streaming diff implementation to its own
`streaming_diff` crate.

It was duplicated between `assistant` and `assistant2`, but their
implementations were exactly the same (and I don't see a reason why they
would need to diverge).

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -9,7 +9,6 @@ mod context_strip;
 mod inline_assistant;
 mod inline_prompt_editor;
 mod message_editor;
-mod streaming_diff;
 mod terminal_codegen;
 mod terminal_inline_assistant;
 mod thread;